Increased physiological performance of the calcifying green macroalga Halimeda opuntia in response to experimental nutrient enrichment on a Caribbean coral reef

Calcifying green macroalgae of the genus Halimeda are widely distributed on coral reefs and may become more abundant under higher nutrient availability. To determine how nutrient enrichment affects the physiological performance of Halimeda opuntia (Linnaeus) J.V.Lamouroux in relation to different water depths, we carried out in situ nutrient enrichment experiments in Curaçao, Netherlands Antilles. H. opuntia was collected in 5 m and then incubated at 5 and 15 m in clear acrylic cages with or without addition of N and P. Growth, algal tissue composition (internal C, N, and P content, δ15N signatures, protein content), photosynthetic performance and pigment content were measured after 14 days of incubation. Growth rates and total C increased with nutrients and were higher in 5 m water depth. N and P content were higher and δ15N signatures were lighter with nutrients in both depths. Photosynthetic performance, concentrations of the main and accessory photosynthetic pigments, and photoprotection also increased with nutrients and showed some response to depth. These results indicate that nutrient enrichment supported a rapid increase in physiological performance of H. opuntia, but with differences in depth. In 5 m, more C was allocated to growth, where light levels were sufficient, while in 15 m C was allocated to photosynthetic pigments. These results suggest that nutrient enrichment may influence their abundance and depth distribution on the reef.

► In situ nutrient enrichment enhances physiological performance of Halimeda.
► Growth rates are highest with nutrient enrichment in shallow depths.
► C, N, and P tissue content increase with nutrients.
► Photosynthetic performance and pigment concentrations increase with nutrients.
► Photoprotection is enhanced with nutrient enrichment in shallow depths.